1. Field of the Invention
The present invention relates to a numerical control apparatus for an NC machine tool detecting a cutting load on the basis of an electric current for driving that is output to a feed drive system and/or an electric current for driving that is output to a spindle drive system, controlling an operation of the feed drive system to be retracted when the detected cutting load exceeds a specified reference value.
2. Related Art Statement
When drilling a deep hole in a work piece by making use of a drill and the like, a drilled hole is stuffed with chips as shown in FIG. 6 when chips are not finely exhausted from the hole, a cutting load rapidly increases as a result, consequently a tool is broken. Here in FIG. 6, a torque computed from the electric current for driving supplied to the spindle drive system is grasped as such cutting load.
Conventionally for removing above-mentioned problems, a retracting operation (or a pecking operation) is applied to the feed drive system before the cutting load will increase too much to break the tool, for exhausting the chips from the drilled hole through the retracting operation of the feed drive system so as to reduce the cutting load. Following three embodiments have been well known to apply the pecking operation to the feed drive system.
In a first embodiment provided with a process section for executing such pecking operation, a pecking process is sure to be executed non-selectively according to a pecking execution code commanded in a machining program. Here, the pecking process is certainly executed irrespective of the actual cutting load or even when the cutting load has not yet reached a load (an overload) which sure needs the pecking operation.
In a second embodiment also provided with the same process section for executing the pecking operation, the pecking process is started according to an automatic pecking execution code commanded in the machining program, for making the feed drive system execute the pecking operation only when the cutting load detected by a load detector installed to a tool holder has reached the overload.
In a third embodiment also provided with the same process section for executing the pecking operation, the pecking process is started according to the automatic pecking execution code commanded in the machining program, on the other hand, the cutting load is detected on the basis of the electric current for driving that is output to the feed drive system of the machine tool and/or the electric current for driving that is output to the spindle drive system, for letting the feed drive system execute the pecking operation only when the detected cutting load exceeds a specified reference value and reaches the overload.
However, in above-mentioned three embodiments, there have been following problems described below. That is to say, in the first pecking process according to the first embodiment, the pecking operation is sure to be executed once the pecking execution code is commanded in the machining program even when the actual cutting load has not yet reached the overload or in a condition not needing the pecking process, therefore, an unnecessary air-cutting time (non-cutting process time) increases, consequently making the process itself inefficient. Furthermore, unnecessary retracting operations and retracting amounts with considering safe first lead to an inefficient process as a result because the actual cutting load has not been detected.
In addition, in the second pecking process according to the second embodiment, the cutting load is detected by the load detector installed to the tool holder for executing the pecking process only when the actual cutting load has reached the overload, therefore an effective process is possible, on the other hand, the pecking process itself needs a large cost because of necessity of a special tool holder and an electric circuit.
Furthermore, since the cutting load fluctuates according to a feed rate of the feed drive system and a wear and abrasion of a tool, it is impossible to uniformly set a reference value for judging the above-mentioned overload, whereby a problem happens such that an experiential skill is necessary to set the optimal reference value. One embodiment is shown in FIG. 7, wherein the cutting load increases according to the feed rate. In FIG. 7, with the use of a coated carbide solid drill with an oil hole (MDW085MHK available from Sumitomo Denko Co., Ltd.) having a diameter of 8.5 mm, a work piece of FC250 was drilled at a cutting speed of 150 m/min for formation of holes each having a depth of 26.5 mm. In addition, in FIG. 7, a first peak indicates the cutting load at the feed rate of 0.2 mm/rev, a second peak indicates same at 0.25 mm/rev, a third peak indicates same at 0.3 mm/rev, a fourth peak indicates same at 0.35 mm/rev, and a fifth peak indicates same at 0.4 mm/rev respectively.
As has been described above, the above-mentioned reference value has to be respectively set according to each feed rate of the feed drive system. Furthermore, the setting of the reference value is so complicated with considering the wear and abrasion of the tool. This can be the same in the third pecking process according to the third embodiment wherein the reference value cannot be set uniformly, either.
For solving the above noted problems in the pecking process, the present invention has its principal object the provision of a numerical control apparatus which can judge the overload with using a specified reference value without any affection of the wear and abrasion of the tool even in machining wherein the feed rate changes every moment, and can also securely execute the pecking process.
In the present invention, a numerical control apparatus for an NC machine tool comprising a command generating and distributing section to generate an operation command signal according to a machining program, a feed drive controlling section to control an operation of a feed drive system on the basis of the obtained operation command signal, a spindle drive controlling section to control a rotational operation of a spindle drive system according to the machining program, cutting load detecting means to detect a cutting load on the basis of an electric current f or driving that is output to the feed drive system from the feed drive controlling section and/or an electric current for driving that is output to the spindle drive system from the spindle drive controlling section, and pecking operation controlling means to make the feed drive system retracted if the cutting load detected by the cutting load detecting means exceeds a specified reference value during a drilling process, wherein the cutting load detecting means comprise a bite load (a load on cut starting) detecting section to detect the cutting load at an early stage being in a stationary condition after a tool has contacted with a work piece, and a stationary load detecting section to detect the cutting load after the early stage, the numerical control apparatus for an NC machine tool further comprises pecking judgement means to compute a fluctuation load by reducing the cutting load detected by the bite load detecting section from the cutting load detected by the stationary load detecting section for judging that the feed drive system should be retracted when the computed fluctuation load exceeds a specified reference value, wherein the pecking operation controlling means make the feed drive system retracted on the basis of judgement of the pecking judgement means.
In the present invention, the cutting load at the early stage is detected by the bite load detecting section, on the other hand, the cutting load after such early stage is detected by the stationary load detecting section. In addition, in the pecking judgement section, the fluctuation load is computed by reducing the cutting load detected by the bite load detecting section from the cutting load detected by the stationary load detecting section. When the computed fluctuation load exceeds the specified reference value, or when an overload caused by a stuffing of chips is detected, it is judged that the retracting operation should be provided to the feed drive system, as a result the pecking operation controlling means make the feed drive system retracted.
Thus, in the present invention, the overload is judged upon the fluctuation load which is computed by reducing the cutting load at the early stage from the cutting load in the stationary condition, whereby fluctuation elements depending on a feed rate of the feed drive system as well as on a wear and abrasion of the tool can be removed so as to judge the overload according to the load data depending only upon the stuffing of the chips, consequently the overload is judged upon the specified reference value. This is because the cutting load at the early stage can be recognized almost the same as the cutting load after the early stage or the stationary load, even when the feed rate of the feed drive system changes for each process hole wherein any fluctuation element depending on the stuffing of the chips never exists. In addition, the cutting load at the early stage is recognized almost the same as the stationary load after such early stage, even when the cutting load increases as the wear and abrasion of the tool progresses.
Thus, according to the present invention, an operation which needs skills and has some troublesome represented by a setting of a reference value for each feed rate of the feed drive system as in the conventional pecking process is never requested, whereby the overload can be easily judged.
Further in the present invention, the pecking judgement means sets a reference value for judgement on the basis of the cutting load detected by the bite load detecting section, judging that the feed drive system should be retracted when the cutting load detected by the stationary load detecting section exceeds such reference value for judgement.
According to the present invention, in the pecking judgement means, the reference value for judgement is computed by multiplying the cutting load data detected by the bite load detecting section by a specified coefficient or adding a specified coefficient to the detected cutting load, thereafter it is judged that the feed drive system should be retracted when the cutting load detected by the stationary load detecting section exceeds the reference value for judgement. Thus, by judging whether the retracting operation is necessary to the feed drive system or not on the basis of the cutting load at the early stage, the fluctuation elements depending on the feed rate of the feed drive system as well as upon the wear and abrasion of the tool can be removed, as a result the overload is judged according to the load data depending only upon the stuffing of the chips, consequently functions and effects same as in the invention according to claim 1 can be expected.
Further, in the invention, the bite load detecting section detects that the tool has contacted with the work piece when the cutting load exceeds a specified reference value, recognizing the cutting load after a rate-of-change of the cutting load detected after such contact of the tool and the work piece becomes less than a specified value as the same cutting load at the early stage.
When the tool contacts with the work piece, the cutting load increases. Therefore, it can be detected that the tool has contacted with the work piece when the cutting load exceeds the specified reference value. Furthermore, since a drill edge is shaped into a cone with a specified angle, the cutting load gradually increases after the tool has contacted with the work piece, being in the stationary condition after the whole of the drill edge has contacted with the workpiece. Hence, it can be detected that the whole of the drill edge has contacted with the work piece by monitoring the rate-of-change of the cutting load after the contact and recognizing that such rate-of-change has become less than the specified value.
In addition, in the invention, the numerical control apparatus comprises tool shape data memory means to memorize a shape data of the tool, wherein the bite load detecting section estimates a time taken for the cutting load to be in the stationary condition after the contact of the tool and the work piece on the basis of the shape data memorized in the tool shape data memory means also upon the feed rate of the feed drive system, recognizing the cutting load after the estimated time has elapsed as the same cutting load at the early stage.
As has been described above, the drill edge is shaped into the cone with the specified angle, therefore if the shape and the feed rate of the tool can be recognized, the time taken for the whole of the drill edge to contact with the work piece after the tool has contacted therewith can be also estimated. According to the present invention, the cutting load after the estimated time has elapsed is detected as the cutting load at the early stage.
In addition, the invention comprises abnormality detecting means for a tool judging that the tool is in the abnormal condition when the cutting load exceeds an abnormality detection reference value by comparing such cutting load detected by the stationary load detecting section and/or the bite load detecting section with the abnormality detection reference value. Since the cutting load before the stuffing of the chips will happen increases as the wear and abrasion of the tool progresses, it is possible to judge whether the wear and abrasion of the tool has reached a limit thereof through comparison between the cutting load and the abnormality detection reference value, as a result an alarm warns an operator of the detected abnormality of the tool.
Thus, the limit of the wear and abrasion of the tool can be detected on the basis of the cutting load detected by the stationary load detecting section and/or the bite load detecting section, however, the cutting load increases also by the stuffing of the chips when the abnormality of the tool is judged upon the cutting load detected by above-mentioned stationary load detecting section, whereby it cannot be securely judged whether the increase in the cutting load depends on the wear and abrasion of the tool or on the stuffing of the chips. Therefore as in the invention according to claim 6, a time taken for judgement by the pecking judgement means is set shorter than that by the abnormality detecting means for a tool. Thus, the pecking operation is judged to be necessary to the feed drive section by the pecking judgement means before the abnormality judgement means for a tool will detect the limit of the wear and abrasion of the tool, as a result the pecking operation is executed, consequently an erroneous judgement of the limit of the wear and abrasion can be prevented because the cutting load detected by the stationary load detecting section decreases less than the abnormality detection reference value when the wear and abrasion of the tool hasn""t reached the limit thereof. On the other hand, since the cutting load at the early stage exceeds the abnormality detection reference value when the wear and abrasion of the tool has reached the limit thereof, the cutting load in the stationary condition consecutively detected by the stationary load detecting section also exceeds the abnormality detection reference value, causing that the stationary load never becomes lower than the abnormality detection reference value even in the execution of the pecking operation, whereby the limit of the wear and abrasion can be detected by the abnormality detecting means for a tool before the stuffing of the chips will actually happen. Thus, the wear and abrasion of the tool can be securely detected without any affection of the pecking operation.